The present invention relates to a revolutionary method and apparatus for producing high tensile steel from low and medium carbon steel.
Steel of a high tensile strength is desirable because one can use less steel in a given application in order to achieve a desired level of strength. One known way of increasing tensile strength is through heat treating the steel, i.e. by elevating the steel to a high temperature and by subsequently quenching.
One limitation on heat treating processes for increasing tensile strength is the carbon content of the steel. A high carbon steel, usually considered to be a steel having greater than 0.60 percent carbon, can through rapid quencing be given tensile strength in the range of 150,000 to 200,000 psi, perhaps even more. Medium carbon steels, generally accepted to be steels having 0.30 percent to 0.60 percent carbon, can usually be heat treated to a tensile strength in the range of 100,000 to 150,000 psi. Low carbon steels, those having a carbon content up to 0.30 percent, usually can be heat treated to tensile strengths in the range of 75,000 to 100,000 psi.
Unfortunately, high carbon steel is the most difficult steel to weld. As a result, low carbon steels are more generally used in structural construction applications since they facilitate fabrication by welding. Since the more weldable steels have lower tensile strengths, more steel must be used to give a particular product its desired strength.
William H. Mc Farland has discovered a method of providing a limited variety of high tensile, low carbon steels by first cold rolling steel sheet and then carefully heating the resulting steel to its austenitic range and then rapidly quenching it. U.S. Pat. No. 3,378,360 to William H. McFarland, entitled "MARTENSITIC STEEL", issuing Apr. 16, 1968 on an application filed Sept. 23, 1964. Unfortunately, the resulting martensitic steel is available commercially only in sheets of very thin gauges i.e. 0.007 to 0.035 inches. The McFarland patent indicates that the sheets can be as thick as 0.100 inches, but even this is much too thin for many applications.
Also for many applications, sheet steel is not usable and in many other applications, thicker steel is required. For as long as man has worked with steel, there has been a need for a method and apparatus for producing a wide variety of sizes and shapes of high tensile, low carbon steel.